1. Field of Invention
This invention relates generally to electromagnetic flowmeters in which a pair of electrodes is mounted at diametrically-opposed positions on a flow tube through which passes the fluid being metered to intersect the lines of magnetic flux produced by an electromagnet, and in particular to a meter of this type in which the electromagnet is so driven as to produce a flux field having a triangular wave form, the resultant electrode flow rate signal being a composite wave that includes a spurious voltage component which is rejected in the converter coupled to the electrodes to yield an output signal that is accurately proportional to the flow rate of the fluid.
2. Status of Prior Art
In an electromagnetic flowmeter, the liquid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the flowing liquid intersects this field, a voltage is induced therein which is transferred to the electrodes to provide an electrode signal. This signal, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed in a converter to actuate a recorder or indicator.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Though A-C operation as disclosed, for example, in the Cushing U.S. Pat. No. 3,693,439 is clearly advantageous in that polarization is obviated and the A-C flow induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications. The two spurious voltages that normally are most troublesome are:
1. stray capacitance-coupled voltages from the coil of the electromagnet to the electrodes, and
2. induced loop voltages in the input leads. The electrodes and leads in combination with the liquid extending therebetween constitute a loop in which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by electrostatic shielding and by low-frequency excitation of the magnet to cause the impedance of the stray coupling capacitance to be large. But the spurious voltage from the second source is much more difficult to suppress.
The spurious voltage resulting from the flux coupling into the signal leads is usually referred to as the quadrature voltage, for it is assumed to be 90.degree. out of phase with the A-C flow-induced voltage. Actual tests have indicated that this is not true in that a component exists in-phase with the flow-induced voltage. Hence, that portion of the "quadrature voltage" that is in-phase with the flow-induced voltage signal constitutes an undesirable signal that cannot readily be distinguished from the flow-induced signal, thereby producing a changing zero shift effect.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi./dt), serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change in the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent and zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But, as previously noted, d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered.
In the patent to Mannherz et al., U.S. Pat. No. 3,783,687, there is disclosed an electromagnetic flowmeter in which the excitation current for the electromagnetic coil is a low-frequency wave serving to produce a periodically-reversed steady state flux field, whereby unwanted in-phase and quadrature components are minimized without giving rise to polarization and galvanic effects. This low frequency wave is derived by means of a presettable scaler coupled to the standard a-c power line (60 Hz) and is at a frequency in the order of 17/8, 33/4, 71/2 or 15 Hz. A similar flowmeter arrangement having a low-frequency square wave drive is disclosed in the Schmoock U.S. Pat. No. 4,370,892.
When the fluid being metered takes the form of a coarse slurry containing solid particles such as sand, fly ash or salt which impinge on the surface of the electrodes as the slurry passes through the meter tube, it has been found that a substantial noise component is generated. This makes signal detection more difficult, and in some instances impossible. The meter electrodes in combination with the fluid acting as an electrolyte define a galvanic cell, and when the solids in the slurry strike the electrodes and alter their interface to the fluid, this action brings about a rapid change in galvanic voltage, thereby generating low frequency noise.
Noise is any voltage that does not convey measurement information. Under the most favorable circumstances where noise has been minimized by filtering or other expedients, there are still certain sources of noise present resulting from the granular nature of matter and energy. While noise fluctuations may be small compared with the total energy transfer involved in most measurements, the existence of a noise background limits the ultimate sensitivity to which a measurement can be carried.
When measuring the flow of slurries, the solid particles therein impinge on the surface of the electrodes in contact with the fluid. The electronic noise resulting from such impingement creates a major problem when the flowmeter is of the type whose electromagnet is driven by a low-frequency square wave; i.e., a frequency which is less than 30 Hz. It has been found that the level of such noise is inversely related to the drive frequency and is therefore relatively high when using a low-frequency drive. Hence by raising the square wave drive frequency, one can reduce the response of the electrode signal converter to noise by the same proportion.
Thus, while an electromagnetic flowmeter operating with a 7.5 Hz square wave drive frequency is subject to a high level of noise when metering slurries, a meter of this type operating at 30 or 60 Hz will almost be free of this noise problem. However, when one increases the frequency of the square wave flux, drive problems begin to develop, for the switching impulse coupled into the electrode circuit and carrying over into the measuring period seriously impairs the accuracy and zero stability of the system. This effect is aggravated by the fact that a larger "zap" voltage is required to overcome the L/R time of the electromagnet coils.
Moreover, at the higher square wave drive frequency, the phase shift or time delay imparted to the flow signal starts to become significant when using the magnet current for a reference voltage, and this gives rise to measurement errors in the converter coupled to meter electrodes.
Inasmuch as the present invention makes use of a triangular wave magnetic flux field, the patent to McHale et al., U.S. Pat. No. 4,513,624, which also uses a triangular magnetic wave in an electromagnetic flowmeter, is pertinent. However, in the McHale et al. patent, the flowmeter makes use of electrodes that are capacitively coupled to the fluid, not in direct contact therewith as in one embodiment of the present invention. Hence, the current signal yielded by the electrodes in this prior patent is not comparable to the voltage signal produced by a meter in accordance with the invention and does not yield similar beneficial results. Moreover, the prior patent does not discriminate between a triangular wave and a rectangular wave component in the signal yielded by the electrodes in the manner of the present invention.